CN1462217A - Electrochemical machining method with optimal machining pulse duration - Google Patents

Abstract

For an electromechanical machining of a work piece there is an optimal pulse duration for the machining pulses corresponding to the maximum copying accuracy. Such an optimal pulse duration corresponds to a certain value of the gap. By alternating the machining pulses with measurement pulses it is possible to obtain an accurate information about the gap dimensions on-line during the electrochemical machining process. The process control means (20) are used to automate the electromechanical machining, while keeping it in the optimal mode. For this purpose the process control means (20) comprise the pulse control unit (26) to establish the pulse duration of the voltage pulses to be applied across the gap (4).

Description

Electrochemical Machining Method with Optimal Machining Pulse Duration

The present invention relates to a method of electrochemically machining an electrically conductive workpiece using an electrochemical machining device comprising a tool electrode opposite a workpiece on a predetermined machining gap filled with an electrolyte, the electrochemical machining device also being included on the machining gap Device for supplying machining voltage pulses.

The invention also relates to an arrangement for electrochemically machining a conductive workpiece by applying an electromachining pulse between the workpiece and a conductive electrode while simultaneously supplying an electrolyte between the workpiece and the electrode.

Electrochemical machining is a process in which a conductive workpiece is dissolved at the location of an electrode while an electrolyte and an electric current are supplied. To this end, the electrode is located in the vicinity of the workpiece, and while electrolyte is fed between the workpiece and the electrolyte, a strong current flows through the electrolyte through the workpiece and the electrode, the workpiece being positive in relation to the electrode. The current is applied in the form of machining pulses with a given amplitude and duration. The electrolyte is changed in the intervals between processing pulses. Under operating conditions, the workpiece dissolves, thus increasing the size of the gap between the workpiece and the electrode. To compensate for this, the electrode and the workpiece are moved relative to each other at a given feed rate, so that the electrode forms cavities or progressive holes in the surface of the workpiece, the shape of the cavities or holes corresponding to the shape of the electrode. This process is used, for example, to form complex cavities or holes in or for hard metals or alloys. The accuracy with which the shape of the cavity or hole in the workpiece is reproduced corresponding to that of the electrode is critical to the quality of the finished product.

A method for electrochemical machining is known in patent application publication number WO 99/51382. According to known methods, the passivation pulses for depositing the passivation layer on the workpiece are carefully applied in the intervals between the processing pulses. The spatial distribution of the passivation pulses can be controlled by appropriate selection of the magnitude and duration of the passivation pulses. Preferably the resulting passivation layer has a greater thickness at the lateral surfaces of the resulting cavity relative to the passivation layer at the front surface of the cavity. In this case, the dissolution rate will be higher at the anterior surface relative to the lateral surface, leading to better replication accuracy.

A drawback of known methods of improving the replication accuracy is the difficulty associated with the selection of the passivation pulse characteristic value and with the selection of the gap size with respect to the front and lateral surfaces of the cavity in order to obtain a non-uniform distribution of the passivation layer. The formation of the passivation layer is affected by the strength of the local electric field. The inhomogeneity of this electric field due to the curvature of the electrodes and deposits on the cathode surface does not produce operating conditions for optimum replication accuracy.

It is an object of the present invention to provide an electrochemical machining method with a further improved replication accuracy, in which the process can be optimally controlled. To this end, a method of the type defined in the opening paragraph is characterized in that a second number of measuring voltage pulses is alternately applied across the machining gap after applying a first number of machining voltage pulses for a predetermined optimum duration across the machining gap in order to Measure the actual value of this machining gap.

According to the technical solution of the present invention and based on the basic consensus of the electrochemical machining gap, for each predetermined value of the gap, there is a single optimal pulse duration corresponding to the best local replication accuracy. It will be appreciated that maximum replication accuracy can be achieved if the local dissolution efficiency varies significantly, for example in the case of adjacent cavities having respective different depths. This optimal operating condition is valid for a certain value of the gap. Exact information on the size of this gap can be obtained online during electrochemical machining by alternately using machining pulses and measuring pulses. If the gap size measurement shows a value that deviates from the preset value, the gap can be adjusted by returning the gap to the predetermined value or selecting another processing pulse duration corresponding to the optimal pulse duration for the measured actual gap value. to change operating conditions. It is important to mention that if the measurement of the gap value is headed away from the predetermined value, it is preferable to return the system setting to optimum operating conditions by reducing the gap value to the predetermined value.

An embodiment of the method according to the invention is characterized in that the optimum duration of the machining voltage pulse depends on the maximum value of the local coefficient for the predetermined value of the machining gap. This technical feature is based on the consensus that, in the case of adjacent cavities, the largest local coefficient corresponding to the ratio of local dissolution rates leads to the best replication accuracy. It is further understood that the local anodic dissolution rate is given by the local current density value, resulting in a local coefficient (L) given by the ratio of the local current density value J(τ,s ₁ ) as a function of time and the gap value : $L=\frac{J(\mathrm{\τ},{\mathrm{the\; s}}_1)}{J(\mathrm{\τ},{\mathrm{the\; s}}_2)},.......\left(1\right)$ Where: s1 is the gap value corresponding to the first cavity

s2 is the gap value corresponding to the second cavity.

Therefore, to calculate the local coefficient values, information about the temporal behavior of the current density as a function of the gap is fully used. Further details are described below with reference to the figures.

A further embodiment of the method according to the invention is characterized in that the duration of the measuring voltage pulse is greater than the duration of the machining voltage pulse, the duration of the measuring voltage pulse being chosen to be at least sufficient for the current density pulse over the machining gap in order to achieve a total maximum value. This technical solution is based on the basic consensus that the time corresponding to the current density pulse is a function of the time voltage and the absolute value of this gap. For a given value of applied voltage, the time corresponding to the overall maximum of the current density pulse is a direct measure of the absolute value of this gap. As explained in detail with reference to the figures, the optimal pulse duration of the machining voltage pulse is shorter than the time corresponding to the maximum value of the current density pulse. Therefore, the pulse duration of the measuring voltage pulse must be selected such that the current density pulse reaches an overall maximum. Knowing the actual gap size from previous measurements and using information about the relationship between this actual gap size and the corresponding optimal machining voltage pulse, the duration of the measurement pulse can be chosen such that the resulting current density pulse across the gap reaches its overall maximum value. The polarity of the measuring voltage pulse preferably corresponds to the polarity of the processing voltage pulse.

A further embodiment of the method according to the invention is characterized in that the machining gap value is corrected in the event that the machining gap actual value deviates from the machining gap predetermined value. Since the predetermined value of the gap is chosen in order to achieve a better dissolution and surface quality of the workpiece, it is advantageous to correct the deviation of the gap value if the measurement results indicate that the actual value of the gap differs from the predetermined value. This gap correction can be accomplished using electrode drive means available in electrochemical machining devices.

Another embodiment of the method according to the invention is characterized in that the interval between machining voltage pulses is set at a value sufficient to replace the electrolyte in the machining gap. According to this technical solution, it is ensured that the operating conditions in this gap are completely restored. It was found that under operating conditions comprising an electrolyte of 5% NaCl and a gap value of 30 μm, an applied voltage of 50 V, and an electrolyte pressure of 300 kPa, the magnitude of the current density was recovered after 300 μs, while the pulse shape was recovered after 600 μs. It was further found that after 600 μs of electrolyte replacement, the electrolyte flow rate was equal to 3 m/s.

A further embodiment of the method according to the invention is characterized in that the value of the interval between the machining voltage pulses depends on a system parameter comprising the amplitude of the current density pulse at the machining gap and the elapsed time for the current density pulse to reach this amplitude time. It has been found that reducing the spacing between processing pulses first results in a difference in dissolution rate along the electrolyte flow, followed by two separate regions. Each new processing pulse is applied in a new electrolyte in the first region, and each new processing pulse is applied in the heated electrolyte comprising the gas phase in the second region. This phenomenon causes pitting corrosion in the second area, impairing the surface quality of the workpiece. Thus, it turns out that for selecting the interval between the machining pulses a system parameter comprising the amplitude of the current density pulse across the machining gap and the time elapsed for the current density pulse to reach this amplitude is used more efficiently.

The invention also relates to an arrangement for electrochemically machining a conductive workpiece by applying an electrical machining pulse between the workpiece and the conductive electrode while supplying an electrolyte between the workpiece and the electrode.

These and other aspects of the invention will be described with reference to the drawings.

Figure 1 shows a schematic diagram of current density pulses on the gap as a function of the voltage applied to the gap;

Figure 2 represents an equivalent circuit in order to represent a discharge circuit for an electrochemical cell;

Figure 3 shows a graph comparing the calculated current density pulses in the gap with the measured values;

Figure 4 shows the optimum duration of the machining pulse and the location of the overall maximum of the current density pulse as a function of the gap size value;

Figure 5 shows the shape of the current density pulse in the gap as a function of the period of the applied voltage pulse;

Figure 6 shows a schematic diagram of an arrangement for electrochemically machining a conductive workpiece;

Fig. 7 shows a schematic functional block diagram of the process control device.

The characteristic current density pulses are given in Fig. 1 for a fixed gap value and for different amplitudes of the rectangular voltage pulses. Increasing the absolute value of the applied voltage results in an increase in the maximum value of the current density pulse, which is reached in a shorter time. As shown in Figure 1, the extrema in the current density pulse, and in particular the overall maximum, increases beyond a certain value of the applied voltage pulse. This overall maximum is due to two opposing phenomena in this gap: first, high heating in the electrolyte, and second, continuous growth of the gas phase in the electrolyte. It is further understood that the absolute value of the overall maximum value of the current density pulse is determined by the absolute value of the voltage pulse and the gap value. The influence of these two factors is dominant for small gap sizes and high amplitudes of voltage pulses, for example, the gap size is in the range of s=20-30 μm, and the applied voltage is about 90V.

Figure 2 shows an equivalent circuit for simulating an electrochemical cell discharge circuit. In the framework of this model, the following assumptions are made:

The electrolyte is a two-phase continuous medium;

The gas phase includes hydrogen produced as a result of H electrolysis;

The gas phase adds additional pressure in the gap due to the viscosity of the electrolyte;

The electrolyte is heated adiabatically;

The electrode potential is constant and equal to its plateau value.

By employing basic electrochemistry knowledge, one skilled in the art can deduce the formula for this current, as shown in Figure 2: $\frac{{\mathrm{di}}_2}{\mathrm{dt}}=\frac{1}{R_a{C}_a}(i_1-i_2),......\left(3\right)$ $\frac{{\mathrm{di}}_4}{\mathrm{dt}}=\frac{1}{R_f{C}_f}(i_1-i_4),......\left(4\right)$ $\frac{{\mathrm{di}}_6}{\mathrm{dt}}=\frac{1}{R_k{C}_k}(i_1-i_6)..........\left(5\right)$

where U is the applied machining voltage;

R _{a, k} are the equivalent Faraday resistance of the anode and the anode reaction, respectively;

_Rg is the resistance of the electrolyte layer in the gap;

_Rf is the resistance of the oxide layer on the anode;

_Cf is the equivalent capacitance of the oxide layer on the anode;

C _{a, k} are the capacitances of the electric double layer of the anode and cathode, respectively;

_Rc is the resistance of the cable;

_Lc is the inductance of the cable;

φ _{a, k} are the potentials of the anode and cathode, respectively.

According to this model, the current density pulses were tabulated and compared with the experimental results (curves 1, 2 of Fig. 3, respectively). Due to the good correlation between the listed values for current density pulse 1 and the measured current density pulse 2 over the gap, the model can be used to optimize the local coefficient (L). The slope parameter K for the local parameter L can be deduced from the equation given by: $K_L(\mathrm{\τ},\mathrm{the\; s})=-\frac{d\left(J_{\mathrm{cp}}(\mathrm{\τ},\mathrm{the\; s})\right)}{\mathrm{ds}},.....\left(6\right)$ $J_{\mathrm{cp}}(\mathrm{\τ},\mathrm{the\; s})=\frac{\underset{0}{\overset{\mathrm{\τ}}{\∫}}J(t,\mathrm{the\; s})\mathrm{dt}}{\mathrm{\τ}}.......\left(7\right)$

τ denotes the duration of the current density pulse

J(τ,s) represents the current density function obtained from this model.

The slope parameter K can further be used for the optimization of process parameters such as the duration of the machining voltage pulse in order to obtain maximum local coefficients. For example, the optimal pulse duration for a given value of the gap must satisfy the following conditions: $\frac{\&PartialD;K(\mathrm{\&tau;},\mathrm{the\; s})}{\&PartialD;\mathrm{\&tau;}}=0;........\left(8\right)$ $\frac{{\&PartialD;}^{2}K(\mathrm{\&tau;},\mathrm{the\; s})}{{\&PartialD;\mathrm{\&tau;}}^2}<0......\left(9\right)$

Conditions (8) and (9) define the optimal curve in the s-t coordinate system (given by the solid line 1 in Fig. 2) where the local coefficient values for each s-t combination are maximized. Use this curve for a certain value of each machining gap to obtain the optimum machining pulse duration. As shown in Figure 4, for a given gap size, the optimum machining pulse duration given by curve 1 is less than the pulse duration for which the current density pulse is sufficient to reach the overall maximum given by curve 2. This phenomenon is further used in an embodiment of a process control device for an electrochemical machining arrangement.

Figure 5 shows the shape of the current density pulse across the gap as a function of the period of the applied machining voltage pulse. The resulting shape of the current density pulse gives information about the recovery of the electrolyte after the application of the voltage pulse. It has been found that reducing the spacing between processing pulses first results in a difference in dissolution rate along the electrolyte flow, followed by two separate regions. Each new processing pulse is applied in a new electrolyte in the first region, and each new processing pulse is applied in the heated electrolyte comprising the gas phase in the second region. This phenomenon causes pitting corrosion in the second area, impairing the surface quality of the workpiece. Thus, it turns out that for selecting the interval between the machining pulses a system parameter comprising the amplitude of the current density pulse across the machining gap and the time elapsed for the current density pulse to reach this amplitude is used more efficiently. For a gap of 30 μm and an applied voltage of 50 V with electrolyte pressure P=300 kPa, it has been found that the system fully recovers after at least 300 μs.

FIG. 6 shows a schematic illustration of an arrangement 1 for the electrochemical machining of an electrically conductive workpiece 2 by means of electrodes 3 . Arrangement 1 consists of positioning the base 6 of the workpiece 2 , a holder 7 for positioning the electrode 3 and an actuator 8 for moving the holder 7 and the base 6 relative to each other. The base 6 and the actuator 8 are mounted on a chassis 9 with a rigid structure so that the working distance between the electrode 3 and the workpiece 2 is set with high precision. The arrangement also includes a reservoir 10 filled with electrolyte 5 in such a way that a gap 4 formed due to the working distance between electrode 3 and workpiece 2 is filled with electrolyte 5 . In the present case, the electrolyte consists of NaNO3 dissolved in water. Alternatively, other electrolytes can be used, such as NaCl or a combination of NaNO3 and acid. Electrolyte 5 is pumped through gap 4 by means not shown in the figure. With configuration 1 , the workpiece 2 can be machined through the electrolyte 5 in the gap 4 between the electrode 3 and the workpiece 2 by machining voltage pulses from the power supply unit 40 . The power supply unit 40 includes a voltage pulse generator 41 and a controllable switch 43 . When the polarity of the applied machining voltage is correct, this causes material of the workpiece 2 to be removed from its surface and dissolved in the electrolyte 5 located at a small distance between the electrode and the workpiece. The shape of the resulting cavity is thus determined by the shape of the electrode opposite it. The arrangement also includes process control means 20 to determine the actual size of the gap value and to set the gap value back to the predetermined value when the gap value deviates on the one hand and to apply the optimal machining voltage pulse on the other hand and measuring voltage pulses. The duration of the voltage pulses applied by the power supply unit to the gap is determined and controlled by the process control device 20 via a calculation unit not shown in the figure and operating the switch 43 .

FIG. 7 shows a schematic functional block diagram of the process control device 20 . The pulsed power supply generator 21 generates machining voltage pulses of optimum duration corresponding to the predetermined value of the set gap according to FIG. 4 . For current density values in the range of 1000-20000A/cm ² and the duration of the voltage pulse front is less than 1000nc, the technical advantages of electrochemical machining by microsecond duration voltage pulses can be obtained. Preferably a lookup table value known in the art for optimum pulse duration as a function of the gap value is used, which value is stored in a lookup standard table. These predetermined values may be obtained as a result of calibration tests for a given setting, or may be calculated using the models described above. The pulse duration of the applied voltage pulses is controlled by the pulse control unit 26 . For the period of the processing voltage pulse, the pulse control unit 26 sets the optimal voltage pulse duration according to the data in the quick reference table. The pulse control unit also operates switches of a power supply generator not shown in the figure. This voltage pulse is then applied across the gap 22, causing the current density pulse to increase across the gap, as shown schematically in FIG. 1 with the curve having an overall maximum. The current density pulses are detected by a shunt 23 and supplied to a calculation unit 24 . The current density pulses measured by the shunt 23 form a control signal which is further used in the process control device 20 . In order to measure the actual gap size, a measuring assembly 27 is integrated in the process control device 20 . This component determines how often samples are measured. For a time interval corresponding to the measurement sample, the measuring assembly 27 inputs a signal to the pulse control unit 26 in order to increase the duration of the voltage pulse relative to the processing voltage pulse. The duration of the voltage pulse is selected by the calculation unit 24 on the basis of previous gap measurement data and information on the time at which the current density pulse reaches the overall maximum. When a measuring voltage pulse is fed into the gap 22 , the corresponding current density pulse is analyzed by the computing unit 24 and the corresponding actual elapsed time to reach the overall maximum is determined. Based on these data, the calculation unit 24 calculates the actual value of the gap using the data of the quick reference table.

Calculation means 24 calculates the actual value of this gap from a quick reference table including the relationship between the gap value, the optimum pulse duration and the elapsed time for the current density pulse to reach the overall maximum value. Then, the calculation means 24 compares this value with the predetermined value of the gap, and if there is a deviation detected, calculates the gap correction value. In order to correct the deviation of the actual gap value, a correction signal is sent from the calculation unit 24 to the actuator 25 to determine the working distance between the electrode and the workpiece. After this operation is completed, the pulse control unit 26 sets the voltage pulse duration corresponding to the optimum value of the machining voltage pulse and performs electrochemical machining of the workpiece. According to this embodiment, the workpiece can be machined using optimum machining voltage pulses, improving the reproduction accuracy of the final product. Since machining pulses and measuring pulses can be used alternately, operating conditions such as the actual value of the gap can be detected online. Correcting the deviation of the actual value of the gap from the predetermined value leads to the fact that the machining of the workpiece takes place maximally in the optimum mode. This feature enables the construction of automated process control of the type described above to further optimize the in-line process control of electrochemical machining.

Claims (10)

1. method of using electrochemical machining apparatus (1) electrochemistry processing electrically conductive workpiece (2), this device comprises the tool-electrode (3) relative with workpiece (2), this workpiece is being filled with on the predetermined machining gap (4) of electrolyte (5), this electrochemical machining apparatus also is included in the device (40) that machining gap (4) is gone up the pulse of supply machining voltage, described method is characterized in that, alternately applies the measuring voltage pulse of second quantity so that measure the actual value of this machining gap (4) on this machining gap apply the machining voltage pulse of first quantity in the predetermined best duration on this machining gap after.

2. the method for claim 1 is characterized in that, the best duration of this machining voltage pulse depends on because the maximum of the local coefficient of this machining gap predetermined value.

3. method as claimed in claim 1 or 2, it is characterized in that, the duration of this measuring voltage pulse, the current density pulse that the duration of this measuring voltage pulse is chosen on this machining gap was enough to reach this overall maximum at least greater than the duration of this machining voltage pulse.

4. method as claimed in claim 3 is characterized in that, determine the numerical value with corresponding lapse of time of overall maximum of this current density pulse, and the actual value of this machining gap depends on described numerical value.

5. method as claimed in claim 4 is characterized in that, the numerical value of this machining gap departs from the actual numerical value of this machining gap under the situation of predetermined value of this machining gap to be revised.

6. as each described method of above-mentioned claim, it is characterized in that the interval between this machining voltage pulse is set on the electrolytical numerical value that is enough to change in this machining gap.

7. method as claimed in claim 6, it is characterized in that, the numerical value at the interval between this machining voltage pulse depends on a systematic parameter, and amplitude and this current density pulse that this parameter is included in the current density pulse on this machining gap reach the time that this amplitude passs.

8. one kind by applying the electric machining pulse and supplying simultaneously the configuration (1) that electrolyte (5) is sent a telegram here chemical process electrically conductive workpiece (2) between workpiece (2) and electrode (3) between workpiece (2) and the conductive electrode (3), it is characterized in that this configuration comprises:

One electrode (3);

Be used at keeper electrode on the spatial relationship (3) and workpiece (2) so that keep the device (6,7,8,9) in the gap between this electrode and the workpiece;

Be used for electrolyte (5) is fed to the device (10) of gap (4);

One can be connected electrically on electrode (3) and the workpiece (2) so that be the supply of electric power source (40) of this workpiece and electrode supply electric power;

Be used for producing the device (23) of control signal at the measuring voltage impulse duration;

Drive the device (7,25) that is used for keeper electrode on spatial relationship (3) and workpiece (2) so that keep the process control device (20) of the predetermined value in gap (4) according to control signal (23).

9. configuration as claimed in claim 8 (1) is characterized in that, process control device (20) comprises electric power supply control apparatus (26,43) and according to the calculation element (24) of the actual numerical value of control signal (23) calculated gap (4).

10. configuration as claimed in claim 9 (1) is characterized in that, the relation that gap (4), optimal pulse duration and current density pulse reached between the time that this overall maximum passs can obtain in the quick checking standard scale for this calculation element.

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